1. Field of the Invention
This invention relates generally to drill strings for drilling boreholes for the production of hydrocarbons and more particularly to a drilling assembly which utilizes a downhole controllable steering device for relatively accurate drilling of short-radius to medium-radius boreholes. The drilling assembly of the present invention is particularly useful with coiled-tubing operations.
2. Description of the Related Art
To obtain hydrocarbons such as oil and gas, boreholes or wellbores are drilled by rotating a drill bit attached to a drill string end. A large proportion of current drilling activity involves directional drilling, i.e., drilling deviated and horizontal boreholes, to increase the hydrocarbon production and/or to withdraw additional hydrocarbons from the earth""s formations. More recently, demand for drilling short to medium radius wellbores has been increasing. The term xe2x80x9cshort radius wellboresxe2x80x9d generally means wellbores with radii between 12 and 30 meters, while the term xe2x80x9cmedium radius wellboresxe2x80x9d generally means wellbores with radii between 30 and 300 meters.
Modern directional drilling systems generally employ a drilling assembly that includes a drill bit at its bottom end, which is rotated by a drill motor (commonly referred to as the xe2x80x9cmud motorxe2x80x9d) in the drilling assembly. The drilling assembly is conveyed into the wellbore by a coiled tubing. A fluid (xe2x80x9cmudxe2x80x9d) under pressure is injected into the tubing which rotates the drilling motor and thus the drill bit. The state-of-the-art coiled-tubing drill conveyed drilling assemblies usually contain a drilling motor with a fixed bend and an orienting tool to rotate the high side of the drilling motor downhole in the correct direction. The currently available coiled-tubing drilling assemblies (systems) with such orienting tools are typically more than sixteen (16) meters long. Tools of such length are difficult to handle and difficult to trip into and out of the wellbore. Furthermore, such tools require long risers at the surface. Such orienting tools require relatively high power to operate due to the high torque of the drilling motor and the friction relating to the orienting tool.
To drill a short radius or medium radius wellbore it is highly desirable to be able to drill such wellbores with relative precision along desired or predetermined wellbore paths (xe2x80x9cwellbore profilesxe2x80x9d), and to alter the drilling direction downhole without the need to retrieve the drilling assembly to the surface. Drilling assemblies for use with coiled tubing to drill short-radius wellbores in the manner described above need a dedicated steering device, preferably near the drill bit, for steering and controlling the drill bit while drilling the wellbore. The device needs to be operable during drilling of the wellbore to cause the drill bit to alter the drilling direction.
The present invention provides drilling assemblies that address the above-noted needs. In one embodiment, the drilling assembly includes a steering device in a bearing assembly which is immediately above the drill bit. The steering device may be operated to exert radial force in any one of several directions to articulate the drill bit along a desired drilling direction. The steering assembly may be disposed at other locations in the drilling assembly for drilling medium radius wellbores. Devices and/or sensors are provided in the drilling assembly to continuously determine the drilling assembly inclination, azimuth and direction. Other measurement-while-drilling (xe2x80x9cMWDxe2x80x9d) devices or sensors may be utilized in the drilling assembly, as is known in the drilling industry.
The present invention provides a drilling assembly for drilling deviated wellbores. The drilling assembly contains a drill bit at the lower end of the drilling assembly. A motor provides the rotary power to the drill bit. A bearing assembly disposed between the motor and the drill bit provides lateral and axial support to the drill shaft connected to the drill bit. A steering device integrated into the drilling motor, preferably in the bearing assembly provides direction control during the drilling of the wellbores. The steering device contains a plurality of ribs disposed at an outer surface of the bearing housing. Each rib is adapted to move between a normal position or collapsed position in the housing and a radially extended position. Each rib exerts force on the wellbore interior when in the extended position. Power units to independently control the rib actions are disposed in the bearing assembly. An electric control unit or circuit controls the operation of the power units in response to certain sensors disposed in drilling assembly. Sensors to determine the amount of the force applied by each of the ribs on the wellbore are provided in the bearing section. The electric control circuit may be placed at a suitable location above the drilling motor or in the rotating section of the drilling motor.
For drilling short radius wellbores, a knuckle joint or other suitable device may be disposed uphole of the steering device to provide a desired bend in the drilling assembly above the steering device. Electrical conductors are run from a power source above the motor to the various devices and sensors in the drilling assembly.
During drilling of a wellbore, the ribs start in their normal or collapsed positions near the housing. To alter the drilling direction, one or more ribs are activated, i.e., extended outwardly with a desired amount of force on each such rib. The amount of force on each rib is independently set and controlled. The rib force produces a radial force on the drill bit causing the drill bit to alter the drilling direction.
Examples of the more important features of the invention thus have been summarized rather broadly in order that the detailed description thereof that follows may be better understood, and in order that the contributions to the art may be appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims appended hereto.